This invention relates in general to electrostatography and, more specifically, to a process for preparing a photoconductive device.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the electrode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compound. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. However, when the charge transport layer comprises a film forming resin and one or more of certain diamine compound, difficulities have been encountered with these photosensitive members when they are used in high volume, high speed copiers, duplicators and printers. For example, it has been found that when certain charge transport layers comprise a film forming resin and an aromatic amine compound, the dark decay characteristics are unpredictable from one production batch to another. Dark decay is defined as the loss of charge on a photoreceptor in the dark after uniform charging. This unpredictability characteristic is highly undesirable, particularly for high volume, high speed copiers, duplicators and printers which require precise, stable, and predictable photoreceptor operating ranges. Erratic variations in dark decay rate can be unacceptable or at, the very least, require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters such as charging potentials, toner concentration and the like to compensate for different photoreceptor dark decay rates. Failure to adequately compensate for dark decay rate differences can result in copies of poor copy quality. Moreover, such variations in dark decay rate prevent achievement of optimized dark decay properties.
Similarly, photoreceptors utilizing charge transport layers comprising a film forming resin and one or more of certain aromatic amine compounds also exhibit eratic variations in background potential from one production batch to another. Background potential is defined as the potential in the background or light struck areas of a photosensitive member after exposure to a pattern of activating electromagnetic radiation such as light. Unpredictable variations in background potential can adversely affect copy quality, especially in complex, high volume, high speed copiers, duplicators and printers which by their very nature require photoreceptor properties to meet precise narrow operating windows. Thus, like photoreceptors that exhibit batch to batch dark decay variations, photosensitive members that have poor background potential characteristics are also unacceptable or require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters. Inadequate compensation of background potential variations can cause copies to appear too light or too dark. In addition, such variations in background potential properties preclude optimization of background potential properties.
Control of both V.sub.DDP and V.sub.BG of photosensitive members is important not only initially but through the entire cycling life of the photosensitive members.
Thus, the characteristics of photosensitive members comprising a conductive layer and at least two electrically operative layers, one of which is a charge transport layer comprising a film forming resin and one or more aromatic amine compounds, exhibit deficiencies which are undesirable in high quality, high volume, high speed copiers, duplicators, and printers.